(1) Field of the Invention
This invention relates to an electrically heat recoverable article which increases its wall thickness on recovery. One application of the invention is sealing a space between an elongate object and a surface surrounding the elongate object. Another application is an internal seal or coupler for hollow conduits such as pipes.
(2) Introduction to the Invention
A common example where sealing is needed between an elongate object and a surface surrounding the or each object is for ducts and supply lines, where some form of seal therefore has to be provided either around the substrate (for example in the case of a supply line) or between two substrates (for example in the case of a duct seal between the duct and a supply line it carries). Other similar applications also exist for sealing between elongate objects such as pipes and cables and substrates surrounding or carried within the elongate objects. The substrates and elongate objects may have irregular surfaces.
Various techniques have been used in the past for forming such seals. The simplest perhaps is a tape wrap which has the advantage that a single product could be used over virtually any size or shape of substrate, but the disadvantage of poor performance. Sealing materials such as adhesives or mastics may be used alone or with tapes, but again performance is poor.
In recent years dimensionally-recoverable articles have been used for sealing supply lines and as duct seals. In general, a dimensionally-recoverable article is an article the dimensional configuration of which may be made substantially to change when subjected to a suitable treatment, for example heating. Usually these articles recover towards an original shape from which they have previously been deformed but the term also includes an article which adopts a new configuration even if it has not previously been deformed.
In their most common form such articles comprise a heatshrinkable sleeve made from a polymeric material exhibiting the property of elastic or plastic memory as described, for example, in U.S. Pat. Nos. 2,027,962, 3,086,242 and 3,597,372. In the production of heat-recoverable articles, the polymeric material may be cross-linked at any stage in the production of the article that will enhance the desired dimensional recoverability.
An advantage of these articles is that they are strong, impermeable, water resistant and well able to resist such agencies as the small mechanical stresses caused by temperature variation, earth movements and repair work. They can be easily installed in their recoverable configuration and then caused to shrink (or otherwise recover) into sealing engagement with the substrate that they are to protect. In this way, ease of installation can be combined with excellent performance. A single size of product may be used with a variety of sizes of substrate, and close tolerances in manufacture may be avoided.
A difficulty, however, remains in some circumstances. The very fact that these articles recover on heating means that in general heat must be applied when the article is in approximately the desired position with respect to the substrate. This may be a problem where the substrate or some article nearby physically prevents or hinders proper positioning of a torch or other heating means, or where it may be damaged by excessive heat.
European patent application Publication No. 0179657 (B108) describes an article which can be heated away from the desired installation position, and then brought into contact with the surrounding surface by activation of a resilient biasing member.
It is also known for recoverable articles to comprise conductive polymeric materials which can be heated to effect recovery by passing electrical current therethrough. As an example GB No. 1265194 describes a conductive polymeric, shrinkable tubular layer, which is optionally used in conjunction with a non-conductive shrinkable tubular layer. Current is supplied to the conductive layer by alligator clips. The passage of the current heats and hence recovers the article.
It is also known to be desirable to use for a conductive recoverable article, a material which exhibits a PTC (positive temperature coefficient of resistivity) effect, that is a sharp rise in resistivity at a particular, or over a short range of temperatures. This temperature or temperature range is known as the anomaly or switching temperature. Typically the material is selected to exhibit the sharp rise in temperature at or slightly above the recovery temperature of the article. The PTC effect minimises overheating and consequent so-called "thermal runaway" of the material. Materials which exhibit a PTC effect are typically referred to simply as PTC materials.